This invention relates to a tappet for a switchable valve of an internal combustion engine of the type having a hydraulically actuated piston and clutch member for selectively switching valve operation between a valve operating and non-operating position
In the case of known tappets of the above-mentioned construction (PCT Publication WO-93/18 284), it is possible that, during the valve stroke and when pressure is admitted to the piston, no clear position is fixed between the valve stem and the bore in the clutch member, which may have the result that the valve stem dips into this bore with a high wear-prone friction. In addition, although, in the case of a correspondingly unfavorable alignment of the valve stem relative to the bore of the clutch member, the clutch member may take along the valve, this takes place only along a partial area of the stroke height from where, under the effect of the valve spring, the valve strikes back into the valve seat in an undamped manner, which causes very disturbing noises and additional wear.
It is an object of the invention to provide a tappet of the above-mentioned type whose clutch member can be switched reliably and in a defined manner out of its end positions in fixed time periods or in the case of defined cam angles so that a low-wear, reliable and fast switching is permitted.
According to the invention, this object is achieved by providing a tappet for an internal combustion engine valve comprising a tappet member which in use is moved by a cam of an engine camshaft, a clutch member movable between a first switching position transmitting tappet member movements to an engine valve and a second switching position without transmitting tappet member movements to the engine valve, whereby an associated engine valve can be switched between operating and non operating conditions, a selectively movable piston engageable to move said clutch member between said first and second switching positions, and first and second locking devices operable to lock the clutch member in respective end positions assigned to the first and second switching positions,said first and second locking devices being operably acted on during use by a valve stem of the engine valve to effect unlocking of the locking devices.
A high switching reliability of such a tappet is achieved by means of the unlockable locking of the clutch member in two end positions assigned to the switching positions of the valve. By means of the locking of the clutch member in these two end positions, it is achieved that the respective switching operation will be possible only after the unlocking of the respective locking device applied to the clutch member. Because of the fact that the locking devices can each be unlocked by the valve stem, the unlocking of the clutch member is possible as a function of the position of the valve stem and therefore of the angle of rotation of the cam or at a defined point in time. Thus, it can be ensured that the switching operation starts in both directions in each case at a fixed point in time or during a fixed time period. This corresponds to a defined angular position of the cam. Thus, the switching operation is carried out in a reliable and defined manner within a short time. An increased wear as a result of excessive frictional forces or edge pressures is therefore excluded. As a result of the fact that the unlocking of the locking devices of the clutch member takes place by means of the valve stem of the valve, it is possible to use standard camshafts. The valve gear may therefore have a correspondingly simple construction since, in particular, a high-expenditure operating hydraulic system in the rotating system of the camshaft will not be necessary.
The locking of the clutch member in the two end positions becomes particularly reliable if it takes place by the effect of a spring element when the end positions are reached. This permits a quasi-automatic locking in the case of end positions without requiring, beyond the intervention of the valve stem, additional switching or locking mechanisms acting from the outside. The spring elements can be integrated in the tappet in a space-saving manner.
An unlocking of one of the locking devices in the upper area of the stroke curve of the corresponding cam can be achieved in an advantageous manner if the clutch member has a bore into which the valve stem can dip in the case of a corresponding position of the clutch member. If the corresponding locking device is positioned such that it is unlocked by the valve stem in the dipped-in position, an unlocking is achieved which correspondingly is in effect in the upper stroke area. The switching point in time can be determined by way of the dipping length of the valve stem.
An unlocking of the second locking device before passing though the base circle range of the cam is achieved when, in the second end position of the clutch member, the valve stem rests against this clutch member.
The switching operation is advantageously achieved by the displacement of the clutch member if it has spaced supporting surfaces on which the piston is supported on the one hand, and the spring element is supported on the other hand, by means of whose interaction the clutch member can be displaced into its respective end positions (when the locking device is unlocked). In preferred embodiments, the tappet is constructed in the manner of a cup such that the piston, the locking devices, the clutch member and a spring element may advantageously be inserted in the interior so that the tappet requires the same amount of space as conventional cup tappets.
An advantageous form of the locking device according to certain preferred embodiments includes a springy lock bolt constructed in the tappet and applied to the clutch member in the corresponding end position. Another advantageous form of the construction of the locking device is obtained by the interaction of two meshing parts of the clutch member and the guideway guiding this clutch member. In this case, the unlocking advantageously takes place by the lifting of the clutch member relative to the guideway.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.